In this technical field, research and development on next generation mobile communication systems have been accelerated under 3GPP.
FIG. 1 schematically shows an exemplary communication system. In FIG. 1, an access gateway (aGW) 11, base stations (eNBs) 12, 13 connected to the access gateway 11 via interfaces S1, and a user apparatus (UE) 14 wirelessly communicating with the base station 12 or 13 are illustrated. The access gateway (aGW) 11 may be referred to as a mobile management entity/service gateway (MME/S-GW). Typically, the user apparatus 14 may be a mobile station. The base stations 12, 13 are connected with each other via an interface x2. In the illustration, the user apparatus 14 is conducting handover, and the left-hand base station 12 works as a source base station (handover source) whereas the right-hand base station 13 works as a target base station (handover target). The base stations 12, 13 constitute a radio access network (RAN). The access gateway (aGW) 11 may or may not constitute the radio access network (RAN) together. In any case, the access gateway (aGW) 11 is connected with a base station (eNB) and an apparatus on a core network (CN).
Communications between the access gateway (aGW) 11 and the base stations (eNBs) 12, 13 are conducted in a data unit referred to as “service data unit (SDU)” via the interfaces S1. For example, the service data unit may be an internet protocol (IP) packet or include a processing unit such as 1500 bytes or variable data sizes depending on applications. On the other hand, communications between the base stations 12, 13 and the user apparatus 14 are conducted in another data unit referred to as a packet data unit (PDU). The size of the packet data unit (PDU) may be variable depending on radio channel conditions.
FIG. 2 schematically shows exemplary correspondence between service data units (SDUs) and packet data units (PDUs). In the illustration, the horizontal axis corresponds to time. For convenience, downlink data transmissions are explained herein. The data size of the service data units (SDUs) is variable depending on applications. In addition, the data size of the packet data units (PDUs) is also variable depending on radio channel conditions. In radio transmission with keeping a predefined quality, a greater data size may be applied under a better channel while a smaller data size may be applied under a worse channel condition.
In order to restore received signals without error, Hybrid Automatic Repeat request (HARQ), which is a combination of error correction coding and automatic retransmission control, may be utilized. More specifically, N-channel Stop-and-Wait scheme HARQ may be utilized.
FIG. 3 schematically shows an exemplary application of 6-channel Stop-and-Wait scheme. In the illustrated example, a portion of packet transmission to a certain user apparatus is illustrated. For convenience, downlink data transmission is explained herein. In this example, the transmitter side is a base station, and the receiver side is a user apparatus. On the other hand, this role assignment may be reversed in uplink data transmission. Numbers “1” to “6” are cyclically assigned as process IDs. As shown on the left side of the illustration, a packet data unit (PDU) with “#1” (which is simply indicated as “#1” in the illustration) is transmitted in a frame associated with process ID with “1”. In the illustration, transmitted packet data units #1, #2, . . . are illustrated with the same size, but actually transmitted packet data units (PDUs) may have various data sizes depending on channel conditions as stated above. When the receiver side successfully receives packet data unit #1, the receiver (user apparatus in this example) returns acknowledgement (ACK) signal to the base station. Upon receiving the ACK signal, the base station transmits packet data unit #7 (N+1=6+1=7) in the next process ID “1”.
On the other hand, packet data unit #2 transmitted in process ID “2” has not been successfully received, and a non-acknowledgement (NACK) signal is returned to the base station. In response to receipt of the NACK signal, the base station retransmits packet data unit #2 in the next process ID “2” (which is illustrated in the transmitter side as “#2 Retransmission”). Upon receiving the retransmitted packet data unit #2, the user apparatus combines the firstly received PDU #2 with the presently retransmitted PDU #2 and performs error detection on it again. In the illustration, some error is detected again, and accordingly further retransmission is requested.
As stated above, the packet data unit is used as the retransmission unit in HARQ and is variable depending on channel conditions. As described in conjunction with FIG. 2, a single packet data unit (PDU) may fully or partially include one or more service data units (SDUs). For convenience, it is assumed herein that packet data units #1, #3 and #4 are successfully received while packet data units #2 and #5 are unsuccessfully received. Under this assumption, it would be concluded that only one service data unit (SDU #3) is successfully received. In other words, the other service data units (SDU #1, #2 and #4) could not be completed. Subsequently, once some of the incomplete service data units are successfully retransmitted, the service data units are completed and thus become available to desired applications.
It is assumed under the situation where packet data units #1, #3 and #4 have been successfully received but packet data units #2 and #5 have not been successfully received that handover between base stations is initiated as illustrated in FIG. 1. In this situation, the source base station 12 forwards service data units SDU #1, #2 and #4 stored in a transmission buffer to the target base station 13. This data transmission is carried out via the interface x2. Then, the source base station 12 discards packet data units PDU #2 and #5. The user apparatus UE obtains information on service data units SDU #1, #2 and #4 from the target base station 13 again. In other words, a new packet data unit is generated to transmit service data units SDU #1, #2 and #4 depending on channel conditions between the user apparatus UE and the target base station 13 and is transmitted to the user apparatus UE.
Although downlink data transmission has been explained above, in response to initiation of handover between the base stations during uplink data transmission, uplink data transmission is carried out as follows. Similar to the above situation, it is assumed that packet data units #1, #3 and #4 have been successfully received while packet data units #2 and #5 have not been successfully received. In this case, the source base station 12 can generate service data unit SDU #3 from successfully received packet data units PDU #1, #3 and #4. The base station 12 forwards completed service data unit SDU #3 to the access gateway (aGW) 11. Upon initiation of handover, the base station 12 discards all the incompletely received service data units SDU #1, #2 and #4. Thus, they will be transmitted from the user apparatus UE to the access gateway 11 via the target base station 13. Such data operations in handover are disclosed in non-patent document “3GPP TSG RAN TR25.813 v0.9.1”.